A former Citigroup ( C) broker pleaded guilty Friday to a criminal charge arising from the so-called squawk box trading-tips scandal, amid indications that federal prosecutors are expanding their investigation. Ralph Casbarro pleaded guilty in federal court in Brooklyn, N.Y., to one count of conspiracy to commit securities fraud. In August, a federal grand jury indicted Casbarro on charges that he participated in an illegal scheme to permit daytraders to listen in to the internal "squawk box" system used by Citigroup's institutional trading desk. Indicted with Casbarro were Timothy O'Connell and Kenneth Mahaffy Jr., two former Merrill Lynch ( MER) brokers, and David Ghysels, a former Lehman Brothers ( LEH) broker. Mahaffy most recently had worked at Citigroup before being let go when the scandal broke this spring. The eavesdropping allegedly allowed the daytraders at AB Watley and Millennium Brokerage to get valuable information about big block trades by institutional customers. Prosecutors contend the daytraders used the squawk boxes to gather information to engage in front-running, an illegal practice in which a person buys or sells shares ahead of a trade he suspects will move a stock's price. In return for the access, the daytraders allegedly paid bribes in the form of cash or trading commissions, after generating at least $650,000 in gross profit from June 2002 through September 2003. Casbarro's attorney, Larry Iason, would not comment on whether his client was cooperating with the investigation. Casbarro is scheduled for sentencing on Dec. 20. People familiar with the investigation say the timing of Casbarro's guilty plea is curious because it comes amid reports that investigators have questioned other Citigroup brokers who at one time had access to similar institutional trading squawk boxes. Investigators are trying to determine whether other brokers made a regular practice of sharing trading tips with daytrading firms and hedge funds.